android: dpi_scale, scissor, JNI safe-insets, touch input

Four Android UX wins landing together; all verified end-to-end on a
Pixel 7 Pro (board fills width, info-panel text renders, status bar
inset honored, tap-to-select + tap-to-move plays 1. e4).

- AndroidPlatform.init reads density via AConfiguration_getDensity
  (app->config at offset 32) and sets dpi_scale = density / 160. The
  hardcoded 1.0 had been making every logical unit equal one physical
  pixel; ChessBoardView's 520-default size_that_fits fallback then
  rendered at ~half the framebuffer width on the device, and glyphs
  rasterized at literal 11-13 physical pixels were essentially invisible
  on a 2340-tall display.
- gles3.sx set_scissor un-stubbed; with dpi_scale right the renderer
  feeds in valid pixel bounds and the Y-flip math lands inside the
  framebuffer.
- New library/vendors/sx_android_jni/sx_android_jni.c walks
  activity -> window -> decorView -> rootWindowInsets via JNI and
  publishes the system-bar insets. safe_insets() lazy-queries the
  first call after EGL is up (decor view isn't attached at bootstrap).
- sx_android_install_input_handler sets app->onInputEvent; sx-side
  sx_android_input_event translates AMotionEvent DOWN/MOVE/UP/CANCEL
  into existing mouse_down/mouse_moved/mouse_up Events so the chess
  board's tap-to-select + ScrollView drag path Just Works. Coordinates
  divided by dpi_scale so layout-side hit tests match. poll_events
  drains its slice after returning (mirrors the SDL pattern).
- src/imports.zig now routes #import c { #source / #include } paths
  through the same chain as #import (importing dir -> CWD -> stdlib
  search paths). Lets library-owned C helpers like the JNI bridge
  live in sx/library/vendors/ without forcing consumers to vendor a
  copy. Existing CWD-relative consumer layouts (chess's vendors/...)
  still resolve first, so no regression.

86/86 regression tests pass.

src/imports.zig

@@ -202,6 +202,26 @@ pub fn resolveImports(
         if (decl.data == .c_import_decl) {
             const ci = decl.data.c_import_decl;
 
+            // Resolve `#source` / `#include` paths through the same chain
+            // as `#import`: importing-file's directory → CWD → stdlib
+            // search paths. This lets sx-library modules ship their own
+            // C helpers (e.g. the Android JNI insets bridge) without
+            // forcing every consumer to vendor an identically-named copy.
+            if (ci.sources.len > 0) {
+                var resolved = try allocator.alloc([]const u8, ci.sources.len);
+                for (ci.sources, 0..) |raw_src, idx| {
+                    resolved[idx] = try resolveImportPath(allocator, io, base_dir, raw_src, null, stdlib_paths);
+                }
+                decl.data.c_import_decl.sources = resolved;
+            }
+            if (ci.includes.len > 0) {
+                var resolved = try allocator.alloc([]const u8, ci.includes.len);
+                for (ci.includes, 0..) |raw_inc, idx| {
+                    resolved[idx] = try resolveImportPath(allocator, io, base_dir, raw_inc, null, stdlib_paths);
+                }
+                decl.data.c_import_decl.includes = resolved;
+            }
+
             // Parse headers to get synthetic function declarations
             const result = c_import.processCImport(
                 allocator,